NULL

Defined in header <stddef.h>
Defined in header <string.h>
Defined in header <wchar.h>
Defined in header <time.h>
Defined in header <locale.h>
Defined in header <stdio.h>
Defined in header <stdlib.h> 
#define NULL /*implementation-defined*/

The macro NULL is an implementation-defined null pointer constant, which may be.

A null pointer constant may be converted to any pointer type; such conversion results in the null pointer value of that type.

Possible implementation

// C++ compatible:
#define NULL 0
// C++ incompatible:
#define NULL (10*2 - 20)
#define NULL ((void*)0)

Example

#include <stdlib.h>
#include <stdio.h>
int main(void)
{    
    // any kind of pointer can be set to NULL
    int* p = NULL;
    struct S *s = NULL;
    void(*f)(int, double) = NULL;
 
    // many pointer-returning functions use null pointers to indicate error
    char *ptr = malloc(10);
    if (ptr == NULL) printf("Out of memory");
    free(ptr);
}

See also

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http://en.cppreference.com/w/c/types/NULL